Lactate dehydrogenase isozymes from mouse testis and rat testis have been fragmented into small peptides by chemical and enzymatic cleavage. Most of these peptides have been separated and their amino acid sequences determined. One-hundred percent of mouse LDH-X and 84% of rat LDH-X (330 amino acids in each protein) have been sequenced. Immunological properties of LDH-X isozymes from mouse and rat have also been characterized.